Measure the width, height and length of your room, using a tape measure. Write these measurements down on a sheet of paper.
Multiply the length, width and height measurements together to find the total number of cubic feet in your room.
Multiply the number of cubic feet in the room by 5, 6 or 7, depending on the type of room you are heating. Multiply by 5 if your room has a normal amount of windows and doors and is well-insulated. Multiply by 6 if the room has larger windows or more windows than usual and has poor insulation. Multiply by 7 if your room faces north. The number you get is the total number of BTUs your room needs.